Solid - State Lasers

Several fundamental modifications of the basic p-n junction electroluminiscent diode exist, and chief among these is the semiconductor injection laser. The gallium arsenide semiconductor laser was invented in 1962. (Fig.1).

Fig. 2. Construction of typical GaAs semiconductor injection laser.

The gallium arsenide diode consists of a layer of the p-type gallium arsenide and a layer of n-type gallium arsenide. In its simplest form, the injection laser is a direct band-gap LED having an exceptionally flat and uniform junction (the active region) bounded on facing sides by two parallel mirrors perpendicular to the plane of the junction, which provide a Fabry-Perot resonant cavity to produce quasi-coherent laser radiation.

The lasers usually operate at 71 K, or lower. The p-n junction is usually made by diffusing Zn (acceptor) in one side of a Te (donor)-doped GaAs crystal. The entire area of the junctions is of the order of 10 ^{– 4} cm².

When an intense electric current, about 20,000 amperes per square centimeter, is applied to the device, it emits coherent or incoherent light, depending on the diode type, from the junction between the two layers of gallium arsenide.

Induced recombination of holes and electrons produces photons, which stimulate in-phase recombination and photon emission by other holes and electrons.

The mirrors on either end of the active region provide the optical feedback necessary to sustain laser action, and a small fraction of the wave propagating between the mirrors emerges from each on each pass. One end face on many commercial lasers is overcoated with a reflective Au film to cause all the radiation to emerge from only one end of the device and thus enhance radiation efficiency.

The most common and best-developed injection laser utilizes GaAs (905 nm), though many other semiconductors have been used to produce wavelengths ranging from 630 nm (Al_xGa_yAs) to 34 µm (PbSnSe).

The development of a semiconductor laser is one of the most important developments in the rapidly changing field of technology. The advantages of this type of laser - a gallium arsenide (GaAs) diode - are great compared to crystal and gas lasers. Semiconductor lasers approach efficiencies of 100% as compared to a few percent of other types; they are excited directly by electric current while other lasers require bulky optical pumping apparatus; because they are excited by an electric current they can be easily modulated by simply varying the excitation current.